Hormone--receptor interactions: melanotropic activities of the covalent serum albumin complexes with alpha-melanotropin, alpha-melanotropin fragments, and enkephalin

Melanocortin 1 Receptor Agonists Based on a Bivalent, Bicyclic Peptide Framework

We have designed a new class of highly potent bivalent melanocortin receptor ligands based on the nature-derived bicyclic peptide sunflower trypsin inhibitor 1 (SFTI-1). Incorporation of melanotropin pharmacophores in each of the two turn regions of SFTI-1 resulted in substantial gains in agonist activity particularly at human melanocortin receptors 1 and 3 (hMC1R/hMC3R) compared to monovalent analogues. In in vitro binding and functional assays, the most potent molecule, compound 6, displayed low picomolar agonist activity at hMC1R (pEC₅₀ > 10.3; EC₅₀ < 50 pM; pK_i: 10.16 ± 0.04; K_i: 69 ± 5 pM) and is at least 30-fold more selective for this receptor than for hMC3R, hMC4R, or hMC5R. The results are discussed in the context of structural homology models of hMCRs in complex with the developed bivalent ligands.

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

The discovery of the endogenous melanocortin agonists in the 1950s have resulted in sixty years of melanocortin ligand research. Early efforts involved truncations or select modifications of the naturally occurring agonists leading to the development of many potent and selective ligands. With the identification and cloning of the five known melanocortin receptors, many ligands were improved upon through bench-top in vitro assays. Optimization of select properties resulted in ligands adopted as clinical candidates. A summary of every melanocortin ligand is outside the scope of this review. Instead, this review will focus on the following topics: classic melanocortin ligands, selective ligands, small molecule (non-peptide) ligands, ligands with sex-specific effects, bivalent and multivalent ligands, and ligands advanced to clinical trials. Each topic area will be summarized with current references to update the melanocortin field on recent progress. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Evaluation of novel 99mTc(I)-labeled homobivalent α-melanocyte-stimulating hormone analogs for melanocortin-1 receptor targeting

Aiming to apply the multivalency concept to melanoma imaging, we have assessed the in vivo melanocortin type 1 receptor (MC1R)-targeting properties of (99m)Tc(I)-labeled homobivalent peptide conjugates which contain copies of the α-melanocyte-stimulating hormone (α-MSH) analog [Ac-Nle(4), Asp(5), D-Phe(7), Lys(11)]α-MSH4-11 separated by linkers of different length (L(2) nine atoms and L(3) 14 atoms). The MC1R-binding affinity of L(2) and L(3) is significantly higher than that of the monovalent conjugate L(1). Metallation of these conjugates yielded the complexes fac-[M(CO)(3)(k(3)-L)](+) (M is (99m)Tc/Re; 1/1a, L is L(1); 2/2a, L is L(2); 3/3a, L is L(3)), with IC(50) values in the subnanomolar and nanomolar range. The MC1R-mediated internalization of 2 and 3 is higher than that of 1 in B16F1 melanoma cells. Biodistribution studies in melanoma-bearing mice have shown low nonspecific accumulation with a tumor uptake that correlates with IC(50) values. However, no correlation between tumor uptake and valency was found. Nevertheless, 2 displayed the highest tumor retention, and the best tumor to nontarget organ ratios.

Interactions of the melanocortin-4 receptor with the peptide agonist NDP-MSH

Melanocortin-4 receptor (MC4R) has an important regulatory role in energy homeostasis and food intake. Peptide agonists of the MC4R are characterized by the conserved sequence His₆-Phe₇-Arg₈-Trp₉, which is crucial for their interaction with the receptor. This investigation utilized the covalent attachment approach to identify receptor residues in close proximity to the bound ligand [Nle₄,d-Phe₇]melanocyte-stimulating hormone (NDP-MSH), thereby differentiating between residues directly involved in ligand binding and those mutations that compromise ligand binding by inducing conformational changes in the receptor. Also, recent X-ray structures of G-protein-coupled receptors were utilized to refine a model of human MC4R in the active state (R^⁎), which was used to generate a better understanding of the binding mode of the ligand NDP-MSH at the atomic level.

The mutation of residues in the human MC4R—such as Leu106 of extracellular loop 1, and Asp122, Ile125, and Asp126 of transmembrane (TM) helix 3, His264 (TM6), and Met292 (TM7)—to Cys residues produced definitive indications of proximity to the side chains of residues in the core region of the peptide ligand. Of particular interest was the contact between d-Phe₇ on the ligand and Ile125 of TM3 on the MC4R. Additionally, Met292 (TM7) equivalent to Lys(7.45) (Ballesteros numbering scheme) involved in covalently attaching retinal in rhodopsin is shown to be in close proximity to Trp₉.

For the first time, the interactions between the terminal regions of NDP-MSH and the receptor are described. The amino-terminus appears to be adjacent to a series of hydrophilic residues with novel interactions at Cys196 (TM5) and Asp189 (extracellular loop 2). These interactions are reminiscent of sequential ligand binding exhibited by the β₂-adrenergic receptor, with the former interaction being equivalent to the known interaction involving Ser204 of the β₂-adrenergic receptor.

Structure and chemistry of the peptide hormones of the intermediate lobe

The peptide hormones of the intermediate lobe are derived from a common precursor protein and are therefore biogenetically and structurally related. They represent a group of linear, flexible peptides which elicit a variety of physiological response. Structure-activity studies have shown that different segments of adjacent amino acid residues have a specific function (e.g. address, message, potentiation) in the interaction of each of these hormones with its receptor(s). This kind of organization of hormonal information is called sychnologic; it is the basic for the pleiotropic action of the opiomelanocortin peptides, i.e. the ability of related peptides to interact with different types of receptors in different target cells. Labelled peptide hormones with radioactive, fluorescent, or photolabile groups at defined sites are a prerequisite for studying hormone-receptor interaction. Multi-labelled derivatives of alpha-MSH are suitable for degradation and intracellular incorporation studies. Photoaffinity labelling of melanophore receptors with azidophenyl-containing analogues of alpha-MSH produces an irreversible stimulation of pigment cells. Covalent conjugates between peptide hormone receptors. These conjugates exhibit remarkable properties such as superpotency, strongly enhanced receptor affinity and prolonged action.

Biological actions of melanocyte-stimulating hormone

Melanocyte-stimulating hormone (alpha-melanotropin, MSH) may function in a number of diverse physiological roles. MSH stimulates (1) rapid translocation of melanosomes (melanin granules) in dermal melanophores to effect rapid colour change and (2) melanogenesis in normal and abnormal (melanoma) epidermal melanocytes. Both actions involve (1) initial binding of the peptide on the melanocyte membrane, (2) transduction of signal to adenylate cyclase, and (3) increased cytosolic levels of cyclic AMP. Efforts to prepare radioiodinated MSH and analogues for radioreceptor studies using melanoma membranes and intact cells reveal that conventional iodination procedures inactivate the hormone because of oxidative and iodination effects on specific structural components of the peptide. These effects can be circumvented by the use of synthetically tailored MSH analogues. Transduction of signal from receptor to adenylate cyclase requires calcium, but prostaglandin or beta-adrenoceptor stimulation of melanophores does not. The nucleotide and metal ion requirements for mouse melanoma adenylate cyclase activity have been characterized. There is both a transcriptional and translational requirement for MSH stimulation of tyrosinase activity and melanin production in melanoma cells. Melanosome translocation within melanophores is enhanced in the absence of extracellular calcium. A model for the MSH control of melanosome movements suggests a bifunctional, but compartmentalized, role for calcium in the action of MSH.

Dimeric DOTA-alpha-melanocyte-stimulating hormone analogs: synthesis and in vivo characteristics of radiopeptides with high in vitro activity

Dimeric analogs of alpha-melanocyte-stimulating hormone (alpha-MSH) labeled with radiometals are potential candidates for diagnosis and therapy of melanoma by receptor-mediated tumor targeting. Both melanotic and amelanotic melanomas (over-)express the melanocortin-1 receptor (MC1-R), the target for alpha-MSH. In the past, dimerized MSH analogs have been shown to display increased receptor affinity compared to monomeric MSH, offering the possibility of improving the ratio between specific uptake of radiolabeled alpha-MSH by melanoma and nonspecific uptake by the kidneys. We have designed three linear dimeric analogs containing a slightly modified MSH hexapeptide core sequence (Nle-Asp-His-d-Phe-Arg-Trp) in parallel or antiparallel orientation, a short spacer, and the DOTA chelator for incorporation of the radiometal. In vitro, all three peptides were more potent ligands of the mouse B16-F1 melanoma cell melanocortin-1 receptor (MC1-R) than DOTA-NAPamide, which served as standard. The binding activity of DOTA-diHexa(NC-NC)-amide was 1.75-fold higher, that of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was 3.37-fold higher, and that of DOTA-diHexa(CN-NC)-amide was 2.34-fold higher. Using human HBL melanoma cells, the binding activity of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was sixfold higher than that of DOTA-NAPamide. Uptake by cultured B16-F1 cells was rapid and almost quantitative. In vivo, however, the data were less promising: tumor-to-kidney ratios 4 hr postinjection were 0.11 for [(111)In]DOTA-diHexa(NC-NC)-amide, 0.26 for diHexa(NC-NC)-Gly-Lys([(111)In]DOTA)-amide, and 0.36 for [(111)In]DOTA-diHexa(CN-NC)-amide, compared to 1.67 for [(111)In]DOTA-NAPamide. It appears that despite the higher affinity to the MC1-R of the peptide dimers and their excellent internalization in vitro, the uptake by melanoma tumors in vivo was lower, possibly because of reduced tissue penetration. More striking, however, was the marked increase of kidney uptake of the dimers, explaining the unfavorable ratios. In conclusion, although radiolabeled alpha-MSH dimer peptides display excellent receptor affinity and internalization, they are no alternative to the monomeric DOTA-NAPamide for in vivo application.

Synthesis and receptor binding analysis of thirteen oligomeric alpha-MSH analogs

Thirteen oligomeric analogs from dimers up to a hexamer of alpha-melanocyte-stimulating hormone (alpha-MSH) were synthesized and tested on melanoma cells for their ability to bind to melanocortin type 1 (MC1) receptors and to stimulate melanin production in the cells. The peptidic oligomers were made by linking several copies of the alpha-MSH fragment analog Nle-Asp-His-[D-Phe]-Arg-Trp-Lys-NH2 to different templates through formation of oxime bonds. They were found to have binding affinities at 37 degrees C up to 8 times higher and melanogenesis-inducing activities up to 4 times higher than those of the native hormone. At 15 degrees C, one dimer showed a binding affinity 20 times higher than that of alpha-MSH. These results are discussed in terms of possible bridging of neighboring receptors which has been suggested to occur in some other systems.

Antipyretic properties of centrally administered alpha-MSH fragments in the rabbit

alpha-Melanocyte stimulating hormone (alpha-MSH 1-13) has marked antipyretic effects when administered centrally or peripherally in small doses. A C-terminal fragment, alpha-MSH (11-13), contains an antipyretic message sequence of alpha-MSH; however, the lesser potency of this fragment relative to that of the entire molecule suggests that other amino acids of the alpha-MSH sequence are essential for the full antipyretic effect. Graded doses of alpha-MSH (11-13) (Ac LysProVal NH2), alpha-MSH (10-13) (Ac GlyLysProVal NH2), and alpha-MSH (8-13) (Ac ArgTrpGlyLysProVal NH2), were injected into the cerebral ventricles of rabbits made febrile by IV administration of crude interleukin-1. All three fragments reduced fever in a dose-related manner. The (8-13) sequence was much more effective than the other two fragments, and the (10-13) portion was less effective than the (11-13) tripeptide. None of the fragments was as potent as alpha-MSH (1-13). The results confirm that an antipyretic message resides within alpha-MSH (11-13) and sequential addition of amino acids to alpha-MSH (11-13) can both enhance and reduce the potency of the fragment.

Biological activity, binding, and metabolic fate of Ac-[Nle4, D-Phe7]alpha-MSH4-11NH2 with the F1 variant of B16 melanoma cells

The alpha-MSH (alpha-melanocyte-stimulating hormone) agonist, Ac-[Nle4, D-Phe7]alpha-MSH4-11NH2 (hereafter called ND4-11 alpha-MSH), is at least 10-fold more potent than alpha-MSH as a stimulus of tyrosinase activity in F1 variant cells of B16 melanoma. The binding to these cells during an incubation with 5 nM (3H)ND4-11 alpha-MSH at 37 degrees C is maximal at 0-30 min, 22 fmol/10(6) cells, but declines to 40% of this value at 4 hr. in the presence of 5 nM (3H)ND4-11 alpha-MSH at 37 degrees C, the acid soluble (cell surface) radioactivity decreased rapidly from 11.4 fmol/10(6) cells at 5 min to 4.6 fmol/10(6) cells at 4 hr. Chromatographic analysis of media and cellular samples revealed that there was no evidence of degradation of (3H)ND4-11 alpha-MSH in the medium but there was evidence of intracellular degradation of (3H)ND4-11 alpha-MSH. Ammonium chloride (10mM) resulted in an increase in acid resistant radioactivity (internalized hormone) at 4 hr. The binding to F1 variant cells during an incubation with 0.155 nM or 5 nM (3H)ND4-11 alpha-MSH at 4 degrees C was constant from 4 hr to 24 hr. Under these conditions, there was no time-dependent change in the acid soluble radioactivity from 4 to 24 hr. Scatchard analysis of (3H)ND4-11 alpha-MSH binding to F1 variant cells at 4 degrees C demonstrated that there were approximately 4500 receptors per cell and an association constant of 17.1 nM-1. These results are consistent with a process of (3H)ND4-11 alpha-MSH binding to its receptor followed by internalization of the receptor-hormone complex and then intracellular degradation of the hormone.

ACTH1-4 potentiates alpha-MSH-induced melanophore dispersion and excessive grooming

The biological activity and a possible modulatory role of the N-terminal tetrapeptide Ser-Tyr-Ser-Met from alpha-MSH/ACTH was tested in the Anolis melanophore assay, the Xenopus melanophore assay, tyrosinase stimulation in mouse melanoma cells and in excessive grooming in the rat. ACTH1-4 did not exhibit biological activity in any of these four assays nor did it have modulatory properties in the Xenopus and the melanoma cell assay. However, in the Anolis assay ACTH1-4 potentiated pigment dispersion induced by alpha-MSH, alpha-MSH5-13 and ACTH1-24 by a factor of about 2. In the grooming assay ACTH1-4 potentiated the effects of alpha-MSH, alpha-MSH5-13, ACTH1-16 and ACTH5-16, but not those of ACTH1-24. Oxidized ACTH1-4 was without biological activity and potentiating properties in all four assays. This study shows that small fragments of the pro-opiomelanocortin precursor, which are devoid of biological activity, can modulate peripheral and central actions of alpha-MSH/ACTH.